The present invention relates to method and apparatus to determine the integrated exposure of a material or device to optical (typically ultraviolet) radiation. A specific application is to disposable medical devices.
Ultraviolet light is that portion of the electromagnetic spectrum adjacent to the violet end of the visible spectrum, having wavelengths roughly between 380 nm and 10 nm. Such electromagnetic radiation, as well as some portion of visible light, is energetic enough to induce chemical and/or structural degradation in many materials. In vacuum all wavelengths of radiation propagate, whereas in a standard atmosphere roughly only wavelengths longer than 280 nm are transmitted with little loss. Approximately 10 percent of the energy from the sun is in the ultraviolet.
Because the energy of an ultraviolet photon is greater than the binding energy of many materials and chemical compounds, exposure to ultraviolet radiation can have detrimental effects. Human exposure, for example, can lead to sunburn and skin cancer. Exposure of polymers to ultraviolet radiation can lead to brittleness and coloration. Generation of point and extended defects is commonplace. Because these detrimental effects can be unobvious, time-delayed, or otherwise inconspicuous, it is desirable to have a method and apparatus to allow easy and rapid determination of the total exposure to ultraviolet radiation.
Measurement of total exposure to UV light, or UV dosimetry, is typically accomplished using either electronic devices, such as filtered photodetectors attached to integrating electronics, or by monitoring the quantity of a chemical species which is formed or destroyed by UV light. This latter practice is called actinometry.
Many photoreactions having potential for actinometric applications have been described. One example is the photohydrolysis of 3,4-dimethoxynitrobenzene. The product of this photoreaction does not have visible absorption bands or fluorescence emission. As a result, a UV dosimeter based on such reactions must include a spectrophotometric device, an unsuitable level of complexity for the current task.
Reversible photoreactions have been suggested for use in actinometry. However, their use in UV dosimetry adds considerable complexity and inaccuracy in interpretation of the total exposure, as the current state of the sensitive material depends not only on the total UV exposure but also on the history of that exposure and of other conditions which can effect the rate of reversal of the photoreaction. Accordingly, such reactions are not considered suitable for the present need.
Prior attempts have been made to use actinometric chemical reactions as the basis for an inexpensive and robust indicator of ultraviolet exposure. For example, Trumble (U.S. Pat. No. 3,787,687) used a transparent or white film comprising a 1,3,3-trimethylindolinobenzopyrylospiran. These compounds undergo a chemical reaction on absorbing sufficiently energetic photons, in the process becoming highly absorptive in the visible spectrum. The exposure of the film to ultraviolet was then estimated by comparison with a color chart. This system had the favorable feature that the chemical reaction was reversible on heating the film, allowing reuse of the dosimeter. However, the photoactive molecules rapidly degraded on exposure to visible, as well as ultraviolet light, leading to inadequate service life for many applications.
A physically and chemically similar system was proposed by Goudjil (U.S. Pat. No. 5,581,090), who replaced the spirobenzopyran derivatives used by Trumble with spirooxazines chosen for ultraviolet sensitivity and resistance to degradation. He was successful, but the resulting ultraviolet sensitive material is not suited for measurement of integrated exposure to ultraviolet, as the color-producing reaction is rapidly reversible.
To summarize, although numerous actinometric systems for the measurement of ultraviolet light exposure have been proposed, none thoroughly address the desire for a simple, inexpensive, and easy to read ultraviolet dosimeter.
A specific application for such an ultraviolet dosimeter is in disposable optical probes, especially for medical diagnosis. If the disposable probe contains a UV dosimeter, and the apparatus to which the probe attaches emits a pulse of ultraviolet light in the course of the desired measurement, then the UV dosimeter of a new probe will comprise no free dye molecules, whereas the UV dosimeter of a probe which has been used previously will have free dye molecules. In order to prevent overuse of such a probe, the apparatus can be configured so as to look for the color (or the fluorescence) of the free dye molecules prior to operation. If the number of free dye molecules is large enough, the apparatus will refuse to function. A case of special interest is when the possibility of contamination is present, and the number of free dye molecules resulting from one prior use is sufficient to prevent operation.
The present invention seeks to satisfy the aforementioned needs by introducing a new class of UV dosimeter. Various embodiments and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.